6.9.2.1. Critical section

1. Lock a resource

   <critical section starts>

2. Do something with the resource

   <critical section ends>

3. Unlock the resource

If the locking is exclusive, only one process can hold the resource at any given time, which means that all the other processes will have to wait. The code between the locking and unlocking functions cannot be interrupted and can therefore become a service bottleneck. That's why this code section is called critical. Its execution time should be as short as possible.

Even if you use a shared locking scheme, in which many processes are allowed to concurrently access the resource, it's still important to keep the critical section as short as possible, in case a process requires an exclusive lock.

The code opens the file for reading, locks and rewinds it to the beginning, reads all the lines from the file, and prints out the lines that contain the string "foo".

The gensym( ) function imported by the Symbol module creates an anonymous glob data structure and returns a reference to it. Such a glob reference can be used as a file or directory handle. Therefore, it allows lexically scoped variables to be used as filehandles.

Fcntl imports file-locking symbols, such as LOCK_SH, LOCK_EX, and others with the :flock group tag, into the script's namespace. Refer to the Fcntl manpage for more information about these symbols.

If the file being read is big, it will take a relatively long time for this code to complete printing out the lines. During this time, the file remains open and locked with a shared lock. While other processes may access this file for reading, any process that wants to modify the file (which requires an exclusive lock) will be blocked waiting for this section to complete.

We don't need the file to be open while the loop executes, because we don't access it inside the loop. Closing the file before we start the loop will allow other processes to obtain exclusive access to the file if they need it, instead of being blocked for no reason.

Since we want to read the file, modify it, and write it back without anyone else changing it in between, we open it for reading and writing with the help of "+>>" and lock it with an exclusive lock. You cannot safely accomplish this task by opening the file first for reading and then reopening it for writing, since another process might change the file between the two events. (You could get away with "+<" as well; please refer to the perlfunc manpage for more information about the open( ) function.)

Next, the code prepares the lines of text it wants to prepend to the head of the file and assigns them and the content of the file to the @lines array. Now we have our data ready to be written back to the file, so we seek( ) to the start of the file and truncate( ) it to zero size. Truncating is necessary when there's a chance the file might shrink. In our example, the file always grows, so in this case there is actually no need to truncate it; however, it's good practice to always use truncate( ), as you never know what changes your code might undergo in the future, and truncate( ) doesn't significantly affect performance.

Finally, we write the data back to the file and close it, which unlocks it as well.

6.9.2.2. Safe resource locking and cleanup code

You can use system utilities to observe the opened and locked files, as well as the processes that have opened (and locked) the files. On FreeBSD, use the fstat utility. On many other Unix flavors, use lsof. On systems with a /proc filesystem, you can see the opened file descriptors under /proc/PID/fd/, where PID is the actual process ID.

However, file-descriptor leakage is nothing compared to the trouble you will give yourself if the code terminates and the file remains locked. Any other process requesting a lock on the same file (or resource) will wait indefinitely for it to become unlocked. Since this will not happen until the server reboots, all processes trying to use this resource will hang.

Is it safe code now? Unfortunately, it is not. If the user aborts the request (for example, by pressing the browser's Stop or Reload buttons) during the critical section, the script will be aborted before it has had a chance to close( ) the file, which is just as bad as if we forgot to close it.

In fact, if the same process runs the same code again, an open( ) call will close( ) the file first, which will unlock the resource. This is because IN is a global variable. But it's quite possible that the process that created the lock will not serve the same request for a while, since it might be busy serving other requests. During that time, the file will be locked for other processes, making them hang. So relying on the same process to reopen the file is a bad idea.

$fh is still a global variable, and therefore the code using it suffers from the same problem.

If you use this approach, please don't conclude that you don't have to close files anymore because they are automatically closed for you. Not closing files is bad style and should be avoided.

Note also that Perl 5.6 provides a Symbol.pm-like functionality as a built-in feature, so you can write:

open my $fh, ">/tmp/foo" or die $!;

and $fh will be automatically vivified as a valid filehandle. You don't need to use Symbol::gensym and Apache::gensym anymore, if backward compatibility is not a requirement.

You can also use IO::* modules, such as IO::File or IO::Dir. These are much bigger than the Symbol module (as a matter of fact, these modules use the Symbol module themselves) and are worth using for files or directories only if you are already using them for the other features they provide. Here is an example of their usage:

use IO::File;
use IO::Dir;
my $fh = IO::File->new(">filename");
my $dh = IO::Dir->new("dirname");

Alternatively, there are also the lighter FileHandle and DirHandle modules.

If you still have to use global filehandles, there are a few approaches you can take to clean up in the case of abnormal script termination.

If you are running under Apache::Registry and friends, the END block will perform the cleanup work for you. You can use END in the same way for scripts running under mod_cgi, or in plain Perl scripts. Just add the cleanup code to this block, and you are safe.

For example, if you work with DBM files, it's important to flush the DBM buffers by calling a sync( ) method:

END {
    # make sure that the DB is flushed
    $dbh->sync( );
}

Under mod_perl, the above code will work only for Apache::Registry and Apache::PerlRunscripts. Otherwise, execution of the END block is postponed until the process terminates. If you write a handler in the mod_perl API, use the register_cleanup( ) method instead. It accepts a reference to a subroutine as an argument. You can rewrite the DBM synchronization code in this way:

$r->register_cleanup(sub { $dbh->sync( ) });

This will work under Apache::Registry as well.

Even better would be to check whether the client connection has been aborted. Otherwise, the cleanup code will always be executed, and for normally terminated scripts, this may not be what you want. To perform this check, use:

$r->register_cleanup(
  # make sure that the DB is flushed
  sub {
      $dbh->sync( ) if Apache->request->connection->aborted( );
  }
);

Or, if using an END block, use:

END {
    # make sure that the DB is flushed
    $dbh->sync( ) if Apache->request->connection->aborted( );
}

Note that if you use register_cleanup( ), it should be called at the beginning of the script or as soon as the variables you want to use in this code become available. If you use it at the end of the script, and the script happens to be aborted before this code is reached, no cleanup will be performed.

For example, CGI.pm registers a cleanup subroutine in its new( ) method:

sub new {
  # code snipped
  if ($MOD_PERL) {
      Apache->request->register_cleanup(\&CGI::_reset_globals);
      undef $NPH;
  }
  # more code snipped
}

Another way to register a section of cleanup code for mod_perl API handlers is to use PerlCleanupHandler in the configuration file:

<Location /foo>
    SetHandler perl-script
    PerlHandler        Apache::MyModule
    PerlCleanupHandler Apache::MyModule::cleanup( )
    Options ExecCGI
</Location>